Oil sands have become an attractive source of oil to support global demand for oil. Oil sands are large deposits of naturally occurring mixtures of bitumen, water, sand, clays, and other inorganic materials found on the earth's surface. Bitumen is a highly viscous form of crude oil. The largest oil sands deposits are found in Canada and Venezuela. In particular, the Athabasca oil sands deposit is equivalent to 1.6 to 2.7 trillion barrels of oil, and is located in the Canadian provinces of Alberta and Saskatchewan. About 10% of the Athabasca oil sands deposit can be mined. Once the oil sands are mined, it is processed by extracting the bitumen.
The bitumen must be extracted and separated from the water, sand and fine clays of the oil sands. Today, the oil sands are mined, crushed, then mixed with hot water, and optionally chemicals, to facilitate extracting the bitumen from the sand and clay fines. The extracted bitumen is separated from the sands and fine clays and is then refined. The remaining sand, fine clays and water, commonly referred to as “tailings”, are further processed to dewater the sand and fine clays. The sand and clay fines are typically disposed, e.g., in a tailings pond where the fine particulates settle, in a dewatering step, to become mature fine tailings. Mature fine tailings are a stable slurry comprising clay, fine sands, water and bitumen. Mature fine tailings have no strength, no vegetative potential and can be toxic to animal life, so the tailings must be confined and prevented from contaminating water supplies. The recovered water from the dewatering step may be re-used in the extraction process. Faster recovery of the water reduces heat energy requirements when this water is recycled for use in the extraction process.
The recovered bitumen from this process is in the form of a froth. The froth comprises a concentrated bitumen (typically 50% or greater), water, fine sand and clays. The froth is treated in a froth treatment unit, which may use steam (to de-aerate the froth) and a naphthenic or paraffinic solvent to recover a bitumen with greater than 95% purity. A byproduct of the froth treatment process is a froth treatment tailings. The froth treatment tailings comprise water, residual solvent, and fine solids that are primarily smaller than 44 micrometers in size. The froth treatment tailings are typically disposed of in a tailings pond. Froth treatment tailings may contribute to mature fine tailings formation.
Extraction of bitumen from sand and clay fines, as well as dewatering of the sand and clay fines for disposal, are especially difficult for so-called “poor quality ores.” Generally, a poor quality ore, in reference to an oil sands ore is an oil sands ore that contains a large amount of fines that hinder, not only extraction of bitumen, but also the dewatering process of sand and clay fines. Poor quality ores also refer to oil sand ores with low bitumen content and oxidized ores. Poor quality ores are difficult to extract bitumen from at acceptable yields using conventional methods. In addition, more bitumen is retained in the tailings streams from extraction of poor quality ores, which is sent to the tailings pond as a yield loss. The tailings should be essentially free of bitumen and separated from water, so the water can be re-used and the solids can be returned to the environment free of bitumen, within environmental limits. Alberta government guidelines, as per Alberta Energy and Utilities Board Interim Directive ID 2001-7, require mining operations to recovery certain percentage of bitumen, depending on the starting bitumen content of the oil sands ore. For example, ores that contain 10% bitumen, oil producers are required to extract close to 90% bitumen. For poorer ore, such as 7%, oil producers are required to extract at least 55% bitumen. Traditional approaches however have difficulty meeting this government requirement on poorer ores.
Poor quality ores reduce yield by as much as 35 to 50% and are avoided when possible. Alternatively, poor quality ores are blended in limited quantities with good quality ores so they can be processed more effectively. With demand for oil increasing every year, there is a need to mine these poor quality ores and to produce high yield of bitumen. The conventional approach to utilize these poor quality ores is the use of chemical aids to improve bitumen yield
Tipman et al., in U.S. Pat. No. 5,876,592, disclose recovery of bitumen from oil sands in a process comprising adding aqueous caustic to an oil sands slurry, to create an emulsion. The emulsion is allowed to separate into 3 layers, with a top layer of a first froth comprising bitumen, bottom layer, referred to as tailings, comprising water, sand and clay fines that settled, and a middle layer, referred to as middlings, comprising residual bitumen, suspended clay fines and water. The middlings are further processed to recover additional bitumen in the same manner as the oil sands slurry, producing a second froth. The second froth may be combined with the first froth to recover bitumen by dilution with a solvent and removal of sand and clay fines. Tipman's process results in increasing sodium concentrations in the tailings compared to the starting ore.
It has been shown that the presence of monovalent metal ions can reduce bitumen extraction efficiency by Masliyah et al., in Can. J. of Chem. Eng., August 2004, vol. 82, pp 628-654. Masliyah et al., disclose a reduction of bitumen recovery with the presence of 5 mM sodium ions, in the form of NaCl. Masliyah has also discussed the presence of multivalent ions also reduce bitumen extraction efficiency.
Moffett disclosed, in US-2010-0101981-A1, a process for extracting bitumen from an oil sands ore which comprises providing an aqueous slurry of an oil sands ore and contacting the slurry with a polysilicate microgel to produce a froth comprising bitumen and a tailings stream comprising sand and clay fines. Moffett teaches use of silica sols having a low S value, such as an S value of less than 50%. “Low S-value silica sols” are described in European patents EP 491879 and EP 502089. EP 491879 describes a silica sol having an S value in the range of 8 to 45% wherein the silica particles have a specific surface area of 750 to 1000 m2/g, which have been surface modified with 2 to 25% alumina.
Polysilicate microgels contain residual sodium ions from the silica polymerization process. The sodium ions exchange can exchange with the multivalent ions present on the clays such as calcium and magnesium ions. Multivalent metal ions are known to reduce extraction efficiency of bitumen from oil sand ores. It is desirable to reuse the water in the extraction process to reduce the amount of fresh water needed for bitumen extraction. When the water containing these multivalent ions is used in a bitumen extraction process, it is known to reduce the bitumen extraction efficiency.
Alkali metal silicate solutions are distinct from colloidal silica sols by their ratio of silica to metal oxide (SiO2:M2O). For example, solutions of sodium silicate have SiO2:Na2O of less than 4:1, as disclosed by Iler, “The Chemistry of Silica”, Wiley Interscience (1979), page 116. Iler further recited that “silicate solutions of higher ratios were not available.”
Acidified sodium silicate solution has been used to enhance bitumen extraction by Masliyah, et al., Ind. Eng. Chem. Res., 2005, vol. 44, pp. 4753-4761. Although divalent metal ions can be sequestered by the addition of acidified silicates, the monovalent metal ions from the silicate solution are still present during extraction. There is a similar disadvantage with this process as found in WO 2005/028592, that is, solids are dispersed.
Li, et al., Energy & Fuels, 2005, vol. 19, pp. 936-943 disclose the effect of a hydrolyzed polyacrylamide (HPAM) on bitumen extraction and tailings treatment of oil sands ores. Although this process does not result in increased metal ions, careful control of HPAM dosage is necessary to achieve efficiency in both bitumen extraction and in flocculation of solid fines.
Under most conventional extraction processes, the monovalent metal ions are building in concentration, leading to reduced bitumen recovery or need to dilute or treat the recycle water. There is a desire to have reduced metal ions present during the extraction of bitumen. There is also a need to provide a simple, robust process for bitumen extraction wherein the water can be reused for future bitumen extractions without the need for excess fresh water or expensive capital investments to deionize the water. The present invention meets these needs.